In braking a vehicle, for example, a utility vehicle or a trailer, it is advantageous to achieve the shortest possible braking distance while, at the same time, maintaining vehicle stability. For this purpose, the adhesion between the vehicle wheels and the road surface should be utilized to a maximum degree while avoiding wheel lock.
Adhesion as used herein refers to that force which can be transmitted between the wheels and the road surface without the wheels locking. Wheel lock brings about a vehicle state which can be controlled only with difficulty and which is to be avoided. Adhesion between the wheels and the road depends on the frictional force between the contact faces of the wheels and the road surface, which force is, in turn, dependent on the acting weight force. On identical road surfaces, a wheel which is subject to a heavy load, for example, owing to an unevenly distributed cargo, will transmit a larger braking force to the road than one which is subject to a lighter load, and, consequently, a higher degree of adhesion occurs at the wheel which bears a heavier load.
In order to achieve maximum braking effect in, for example, a case of full braking, each wheel should, ideally, be braked in accordance with its instantaneous maximum adhesion. However, this requires that the load on each wheel be known and that each wheel can be braked individually with a corresponding braking force. Since brakes which can be actuated in a mainly pneumatic fashion are used in the construction of utility vehicles, a separate valve for providing the brake pressure which is different for each wheel has to be made available for each brake, and this makes the corresponding braking system very complex and more costly.
Nevertheless, in order to be able to brake with the best possible braking force, it is expedient to detect the load on each axle, rather than the load on each wheel individually, and to calculate therefrom a braking force by which all the wheels of the respective axle are braked. This is based on the knowledge that differences in loading or the adhesion between the wheels of an axle are significantly smaller than such differences between wheels on different axles. It is therefore acceptable to apply the same braking force to the wheels on one axle. In this way, the corresponding braking system can be made simpler and, therefore, more cost-effective.
With respect to axle loads, a distinction is made between a static component, which is present when the vehicle is stationary or when travel is occurring without braking and without acceleration, and a dynamic component, which is superimposed on the static component when the vehicle is operating. The static component is determined essentially by the design and the geometry of the vehicle and the cargo. The dynamic component is determined, in particular, by acceleration processes and deceleration processes. The sum of the static and dynamic components of the axle load is referred to as the instantaneous axle load, which is dependent on acceleration and deceleration and has a significant influence on the instantaneous adhesion, which is relevant for braking processes.
When a vehicle, which has two axles and a height of the center of gravity above the axle, is decelerated, the front axle bears a greater load, while the rear axle is relieved of its load by the same degree. This is dependent on the deceleration value, the static axle loads of the front and rear axles and the geometric properties of the vehicle (for example length and position of the center of gravity of the vehicle).
In order to be able to brake the vehicle optimally, knowledge of the instantaneous axle load of each vehicle axle is necessary. One way to detect the instantaneous axle load is by means of axle load sensors mounted on the axles. However, this is complex and expensive. A simplified approach is to determine the instantaneous axle load of just one axle. For this purpose, a signal which corresponds to the static axle load is detected during travel without braking and without acceleration, and another signal which corresponds to the instantaneous axle load is detected during braking. The instantaneous axle load of the other axle, which is not provided with an axle load sensor, can be extrapolated from the difference between the instantaneous axle load and the static axle load if the distribution of the static axle loads between the front axle and the further axle is known. Such distribution is generally known since it plays a significant role in drawbar trailer design. Alternatively, such distribution could be detected by means of a testing device, for example, in a workshop. However, the instantaneous axle load of one axle can be inferred in vehicles with two axles only if the instantaneous axle load of the other axle is known; if further axles are present, it is necessary to use further axle load sensors.
One possible way of dispensing with further axle load sensors is described in DE 197 07 210. According to DE 197 07 210, the determination of the braking forces is carried out using an axle load sensor, but the rotational speeds of the vehicle wheels are additionally taken into account, instead of detecting the instantaneous axle load of the further axles by means of further axle load sensors. What is referred to as a difference in slip, which is used to detect the braking force, is detected on the basis of the differences between the rotational speeds of the individual vehicle axles. In this context, each wheel is assigned an optimal braking pressure, which makes the braking system complex.
Preferably, a 4S/3M configuration is used for drawbar trailers. Two wheel sensors and one electronic braking system (EBS) valve are arranged on the steering axle. An EBS valve can automatically modulate brake pressure. The wheel which first exhibits a tendency to lock, dominates the anti-lock braking system (ABS) control of the respective axle. This control is carried out according to the principle of modified axle regulation (MAR). One ABS modulator and one pressure regulating duct of an EBS modulator are used for individual regulation of the individual wheels on each further axle. Conventional braking systems of this type use three-channel brake pressure control and three-channel ABS control, as a result of which the costs are not competitive.
WO 03/011664 describes a braking system for utility vehicle trailers which have steerable front axles. In this braking system, the number of axle load sensors can be reduced since the system can also be operated without the front axle having an axle load sensor (pressure sensor). A disadvantage of a braking system of this type is that the front axle cannot be electronically braked if a pneumatic brake pressure is not made available by the towing vehicle. With such a system it is not possible to carry out stability control since this requires the wheels to be able to be actuated independently of the driver's request.